experiences with riverbank filtration and infiltration in germany
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Experiences with riverbank filtration
and infiltration in Germany
Carsten K. Schmidt, Frank Thomas Lange, Heinz-Jürgen Brauch, Wolfgang Kühn
DVGW-Water Technology Center (TZW)
Karlsruher Straße 84, D-76139 Karlsruhe, Germany
IntroductionIn Germany, groundwater is used for drinking water production wherever possible. Whencompared with surface water, groundwater is well protected against most types of pollution, isof relatively regular quality and temperature, and its abstraction can be easily adjusted toshort-term fluctuations in consumption. However, exploitation of groundwater sources isrestricted with regard to quantity. In Germany, this limitation is not given to such an extentfor surface waters. However, surface water, particularly river water, is exposed to dangers ofpermanent and sudden pollution by wastewaters or to disturbances due to storage, transport orapplication of water-endangering substances, thereby always reflecting its function asreceiving water. In order to preserve the protective character of groundwater at least partlywhen utilizing surface water for drinking water preparation, surface water is subjected to anunderground passage via bank filtration or artificial groundwater recharge.
To clarify the terms bank filtration and artificial groundwater recharge, the typical historicaldevelopment of water extraction facilities in an intensively exploited river valley is describedin figure 1. Case A shows, how groundwater that is originating from precipitation usuallyflows towards the river and percolates particularly in low-flow periods into the flowing waveof the river. In times of high flow, however, river water infiltrates vice versa into the aquifer.Thus, the flow direction of the groundwater is variable under natural conditions even withoutany anthropogenic extraction of groundwater. In major river bends, at barrage weirs or incases where the river bed is located on an alluvial cone, an infiltration of surface water takesplace permanently. By construction of a production well in the river valley, as shown in caseB, water, that is withdrawn from the production well, comes from slope-sided nativegroundwater as long as the pumping water level is not lowered too much. Increased pumpingaction creates a pressure head difference between the river and the aquifer and induces theriver water to flow through the riverbed towards the pumping well that consequently extractsa mixture of groundwater originally present in the aquifer and bank filtrated surface water
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from the river. The proportions of both kinds of water in the extracted water can varydepending on both extraction rate and river flow.
Fig. 1. Riverbank filtration and infiltration (artificial groundwater recharge).
The time variable origin of the extracted groundwater often causes quality fluctuations. Thedanger of contaminations in the groundwater percolating from the hinterland requiresprotection of its catchment area by allocation of water protection zones. However, even incases where the existing usage type of the given site between riverbank and water catchmentarea does not allow for a consequent application of adequate restrictions in protection zones,water suppliers should not abandon bank filtration as water treatment stage, since it facilitatesthe following water treatment processes in any case.
Natural groundwater was early recognized as being free of pathogens and was, in comparisonto well-processed surface water, more clear, more attractive and had a refreshing taste. Thedrawback was, however, that natural groundwater was rarely available in the amountsnecessary to cover the demand of large cities. The task set was to increase the supply ofnatural groundwater by the infiltration of surface water. Thus, the historical development ofwaterworks located at rivers took place mostly in such a manner that in the first instancenatural groundwater, than mixed groundwater and later on almost pure bank filtrate wasextracted. Since at smaller rivers even the bank filtrate was not sufficient, river water wasimpounded at some sites. Riverbed clogging was overcome by massive ground loosening.With the construction of artificial ditches and side channels further infiltration zones were
Pretreatment
Percolation of natural groundwaterinto the flowing wave of a river
Extraction of slope-sided natural groundwater
Extraction of natural groundwater and bank filtrate
Extraction of bank filtrate, natural and artificial recharged groundwater
Extraction of natural and artificial recharged groundwater (bank filtrate pushed away)
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created. To improve water quality and to achieve easier cleanabilities of infiltration zones, aspecific sand layer was later incorporated in percolation ditches, channels, and ponds. Afurther stage of development was finally the construction of recharge basins similar to thosefound in nearly all artificial groundwater recharge plants nowadays. In these recharge basins,raw water is passed through a filtering medium that consists of a layer of sand. Artificialgroundwater recharge can be used, as shown in case D, in addition to bank filtration, but canalso be employed as protection tool to push away riverbank filtrate (case E). Operation ofrecharge and water catch at a longer distance from the riverbed results in systems that arewidely unaffected by riverbank filtrate interference.
The effectiveness of bank filtration and artificial groundwater recharge has long beenrecognized in Germany. As a consequence of various bacterial diseases caused by drinkingwater from waterworks with direct intake from rivers in the late 19th century (e.g. outbreak ofepidemic cholera in Hamburg in 1892/93), direct extraction of surface water for public-watersupply fell into discredit and was replaced or supplemented by artificial or natural subsoilpassage of river water due to its efficiency in removing microorganisms from the infiltratingsurface water. Nowadays, approximately 16 % of the drinking water in Germany is producedfrom bank filtrate or infiltrate. Because of pollution, direct treatment of river water hasdropped to 1 %. Water suppliers in Berlin produce approx. 75 % of the drinking water bybank filtration and artificial groundwater recharge. In Germany, more than 300 water worksuse bank filtration and roughly 50 plants are based on artificial groundwater recharge. Inparticular, major water suppliers often make use of artificial recharge in drinking waterproduction. The retention time in both techniques may vary from 5 to 100 days and more. Inpractice, riverbank filtration produces a mixture of waters with different retention times.
Fig. 2. Sources used for drinking water treatment in Germany.
As the pollution of the rivers was very low throughout the first half of the 20th century, it waspossible to use bank filtrate for drinking water without further treatment. However, increasingchemical pollution, especially in areas with significant human activities, which may result inhigh concentrations of ammonia, organic compounds, and micropollutants in the river water,necessitated introduction of supplementary pre- and post-treatment steps to build up amultiple-barrier system. A variety of technologies may be applied to treat bank filtrate and
Groundwater(64 %)
Bankfiltrate(16 %)
Dam(9 %)
Spring water(8 %)
River water(1 %)
Others(2 %)
GermanyGermany(total: 5 billion m3/year)
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infiltrate and treatment strategies may be quite different depending on the river water quality.Aeration or ozone may be used to oxidize iron and manganese that are picked up in anaerobicaquifers and activated carbon can be used for adsorption and protection against more-persistent contaminants. Today nearly all water utilities situated along large rivers usegranular activated carbon filters, often combined with ozonation and filtration.
To ensure a sufficient velocity during infiltration, river water destined for artificialgroundwater recharge may require particle removal by flocculation, sedimentation, orfiltration. In order to protect the groundwater against contamination by the infiltrate, evenfurther pretreatment steps such as ozonation and adsorption may be used under specialcircumstances. Infiltration is often applied when the quantity of water by bank filtration is notenough, bank filtration is impossible due to the geological circumstances, or groundwatersources at the river bank are highly contaminated.
Fig. 3. Process scheme development of river water treatment in Germany.
The purification process of the underground passage starts in the infiltration zone in whichsievable suspended matter and sediments, that provide an adsorption power for hydrophobicsubstances, are accumulated. During infiltration of river water, purification processes takeplace that are similar to the self-cleaning properties found in surface waters, but proceed inthe infiltration zone much more intensively. As in the river, the self-cleaning capacity of bankfiltration and artificial groundwater recharge are available free of cost and do per se notrequire any application of treatment chemicals. Thus, these procedures reduce costs andtechnical requirements for achieving distinct water quality standards. The infiltration layercan be characterized as a gelatinous, biological highly active biofilm which consists of algae,bacteria, fungi and protozoa as well as of organic and inorganic particles. During infiltration,
preozonation
flocculation/sedimentation
ozonation
filtration
activatedcarbon filters
flocculation/sedimentation
groundwatergroundwaterenrichmentenrichment
pH control
safety chlorinationsafety chlorination
riverbank filtrationriverbank filtration
activatedcarbon filters
ozonation
filtration
activatedcarbon filters
River
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the water with its dissolved components meets multifaceted biogenous and abiogenoussurface structures that aid in intensifying most of the self-cleaning mechanisms that are inprinciple also present in the free surface water.
However, during long-term usage, the infiltration layer becomes thicker and begins to hamperinfiltration. This is, for instance, caused by the production of biomass during the purificationprocess. Thus, to maintain an acceptable filtration rate, it is necessary to clean the infiltrationzone periodically in either case. In artificial groundwater recharge plants, the clogged zonecan be easily removed from the clogged filter (along with a small amount of sand) byskimming off following drying-up or by suction-cleaning under water. Ultimately, filter bedperformance is improved after each cleaning. Prepurification procedures can extendoperational periods several times.
More problematic is the compaction of the riverbed (colmation) that would sooner or latercause complete termination of the riverbank filtration process. However, certain naturalregeneration processes take place in the riverbed. These are caused by an increase in flowvelocity giving rise to removal and restorage of the clogged layers, an intermittent reversal ofthe flow direction during water drawdown and in part by digging activities of the animal stockat the river bottom.
Effects of riverbank filtration and artificial infiltration
Shock loads
Bank filtration is an excellent protection tool to compensate peak concentrations and shockloads resulting for instance from chemical spills or defects in industrial wastewater plants, ascan be seen in figure 4. The figure depicts a 1986 contamination of 1,2-dichloroethane in theRhine River. The example demonstrates how a short-term peak pollution in the river turnsinto a longer-lasting pollution of very low concentration in the aquifer bank filtrate.
Underground passage reduces the effects of concentration peaks because of the varyingdistances covered by the water molecules from the river to the well. In the production well,the withdrawn water is a mixture of water that left the river at different times within a largeperiod. As a rule of thumb, the waterworks in the Rhine valley calculate that in case of asudden short lasting spill, only about one to five percent of the concentration can be found inthe bank filtrate. Therefore, bank filtration is a safety barrier against high peak concentrationsfollowing accidents.
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Fig. 4. Example of the protection against shock loads by bank filtration
Fig. 5. Example of the concentration compensation by bank filtration
How bank filtration equalizes concentration peaks is also demonstrated in figure 5 showingthe weekly fluctuating chloride concentration profile in the Rhine River due to an industrialeffluent discharge by potash mines operating at the upper Rhine region. The bank filtrationsmoothes out this fluctuating concentration, as shown by the chloride concentration in theproduction well. Since chloride is typically not eliminated during underground passage, theslightly lower chloride concentration in the bank filtrate is a consequence of mixture effectswith native groundwater in the well.
0
10
20
30
40
12.06.86 27.06.86 12.07.86 27.07.86 11.08.86 26.08.86
1,2-
Dic
hlor
o-et
hane
[µg/
L]
river Rhine
bank filtrate
approx. 3 weeks
0
100
200
300
400
15.09.85 30.09.85 15.10.85 30.10.85 14.11.85 29.11.85
chlo
ride
[mg/
L]
river Rhine
bank filtrate
source: Sontheimer (1991)Rhine near Düsseldorf
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Temperature equalization
An underground passage is the only possibility to attain a cost-effective equalization offluctuating surface water temperatures. These days, this is especially relevant for riversreceiving, even during the warm season, cooling water from power stations. In general,temperature equalization is more effective in longer subsoil passages (figure 6). Bank filtrateis usually cooler than surface water in summer and warmer in winter, resulting in a moreconstant water temperature.
Fig. 6. Temperature equalization during underground passage.
Behavior of inorganics
Trace elements such as iron, manganese, and various heavy metals are eliminated duringground passage, mainly by sorption processes. In aerobic aquifers, removal is achieved by ionexchange processes at negatively loaded surfaces of clay minerals, amorphous ferric oxidesand alumina, and organic solid matter. In anoxic aquifers, the removal of metal ions isdominated by precipitation reactions with sulphide. Removal efficiencies for heavy metalsduring riverbank filtration at the River Rhine are presented in Table 1. It is apparent fromthese long-lasting data sets that the percentage removals vary widely for the differentelements, ranging from 0 to 94 %. All in all, interactions with the ground provide aconsiderable retention of heavy metals in subsoil. Furthermore, heavy metals can be removedby ground filtration for a long time and they cannot be easily remobilized with one exception:
15°
1°
11°10°~ 80 days
24°19°
3°0°
1-4 days
~ 50 days
23°
2°
14°
9°
25°
6°
14°
9°~ 10 days
25°
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14°10°
~ 35 days
(DVW�:HVWSKDOLD
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if conditions in the aquifer become anaerobic, iron and manganese undergo chemicalreduction and appear in the water, necessitating their elimination by treatment.
While chloride and sulfate are usually unaffected by ground passage, phosphate can beremoved by this treatment step due to precipitation in the form of calcium, iron or aluminumphosphate in the ground. In aerobic aquifers, ammonium is transferred to nitrate bynitrification in the presence of oxygen. Due to analogous nitrification processes in the river,ammonium concentrations are usually rather low in surface water. However, even lowammonium concentrations cause an extensive oxygen depletion during infiltration. Formednitrate can eventually be denitrified under anoxic conditions.
Tab. 1. Heavy metal removal by riverbank filtration at the lower Rhine*
Concentration in µg/LMetal
Rhine River Bank Filtrate
PercentageRemoval
Zn 180 33 82Cu 31.8 7.5 51Pb 12.6 3.2 75Ni 9.5 4.7 51Cr 7.9 0.5 94Sn 4.5 3.6 20As 4.2 0.3 93Cd 2.0 0.5 75Se 1.8 1.6 11Ag 0.5 0.5 0Hg 0.3 0.2 33Be 0.1 0.1 0
*mean values 1975-1978
Behavior of biological contaminants
Surface waters are often contaminated with pathogenic microorganisms excreted by humans,cattle, and various domestic and wild animals; however, the main sources are discharges ofmunicipal wastewater effluents and runoff of livestock wastes and from fields receivingmanure. Biological contaminants in surface water include protozoa, bacteria, and viruses.Underground passage for the removal of biological contaminants is, in principle, an efficientsystem. During the passage of pathogens through soil, their numbers are reduced by acombination of processes including adsorption to aquifer materials and inactivation.
The removal process is most efficient when groundwater velocity is slow and when theaquifer consists of granular materials with an open pore space for water flow around thegrains what is improving the contact of the organisms with the grain surface. Given sufficientflow-path length and time, microbial contaminants will be removed or inactivated to levelsprotective of public health. Under optimal conditions, underground passage can achieve up to
DVGW-Water Technology Center (TZW), Germany Schmidt CK et al. (2003)
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8-log virus removal over a distance of 30 m in about 25 days. However, efficiency will bediminished by short path lengths, high heterogeneity, coarse matrices, high gradients, andaccompanying high velocities. Thus, to assure an efficient removal of pathogenic organisms,water suppliers should favorably install or establish underground passages with high flow-path lengths and residence times.
Removal of organics
Natural organic matter (NOM) is a complex mixture of dissolved and particulate organicmaterial present in surface waters including humic acids, hydrophilic acids, proteins, lipids,amino acids and hydrocarbons. NOM in surface water is a major concern for water utilities,since it contributes to odor and deterioration of taste in drinking water and is the mainprecursor for disinfection and oxidation byproducts, such as trihalomethanes (THMs) andhaloacetic acids (HAAs), which are potentially carcinogenic.
Many authors reported on the removal potential of bank filtration and artificial infiltration forNOM after monitoring various sum parameters, such as total organic carbon (TOC), dissolvedorganic carbon (DOC), biodegradable organic carbon, ultraviolet absorbance of water at254 nm (SAK) and assimilable organic cabon (AOC).
Figure 7 compares DOC concentrations in river water with those in bank filtrate for awaterworks in the central Rhine area over the past 25 years. In this time the DOCconcentration in the Rhine dropped significantly due to improvements in wastewatertreatment. A corresponding decrease was also found in the bank filtrate. Within the timeinterval depicted, the percentage of the DOC reduction between the river and bank filtrate wasnearly constant at approximately 50 %.
Fig. 7. DOC concentration in river water and bank filtrate.
0
1
2
3
4
5
6
7
1975 1980 1985 1990 1995 2000
DO
C [m
g/L]
river Rhine
bank filtratesource: ARW (1998)
DVGW-Water Technology Center (TZW), Germany Schmidt CK et al. (2003)
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As demonstrated in figure 8 for the mean annual DOC concentrations of bank filtrate from theRhine River, retention time in the aquifer is an important factor controlling the removalefficiency of riverbank filtration. The data clearly demonstrates the favorable effect of higherresidence times on water quality.
Fig. 8. DOC removal in bank filtration with different retention times.
Bank filtration also improves the microbiological quality of the water, which can be measuredas a decrease in the concentration of assimilable organic carbon (AOC), what means thefraction of total organic carbon in water that can be used for microbiological growth andcharacterizes the ability of a water to support bacterial growth. Figure 9 gives an example ofAOC concentrations in the course of various treatment steps in a waterworks.
Fig. 9. Effect of bank filtration on biological regrowth.
0
50
100
150
River Saale Undergroundpassage
Ozonation Activated Carbon
Year
1974 1975 1976 1977 1978
DO
C in
mg/
L
0
1
2
3
4
5
6Rhine River ~ 30 days < 50 days > 100 days
DVGW-Water Technology Center (TZW), Germany Schmidt CK et al. (2003)
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Underground passage caused a clear decrease in AOC. As expected, a subsequent ozonationstep increased the AOC again due to the oxidation of organics, which become morebiodegradable. Finally, biologically active GAC filters (granular activated carbon) causedanother AOC reduction. According to these results, bank filtrate is a fairly biologically stablewater with a lower disinfection or oxidation demand.
Surface water is affected by industrial, agricultural, and domestic pollution. Various organicmicropollutants have been detected in surface waters. The fate of these substances is mainlydetermined by adsorption mechanisms and biological transformations. The biologicalprocesses responsible for their elimination occur predominantly within the first few meters ofinfiltration. Polar organic molecules, such as complexing agents, pesticides, industrialproducts like aromatic sulfonates, pharmaceutical compounds, and personal care products, areof recent concern. However, numerous studies and long-time investigations in Germanydemonstrated the efficiency of bank filtration and artificial infiltration in regard to manyorganic compounds. For pesticide residues, removal efficiencies employing artificialinfiltration can vary between 10 % (atrazine) and 100 % (lindane) depending on the propertiesof the compound (figure 10).
Fig. 10. Removal efficiency of artificial groundwater recharge for pesticide residues.
Figure 11 indicates the removal efficiency of bank filtration for different polarmicropollutants and various sum parameters. It is obvious that many of the target compoundspresent in the Rhine River water are eliminated during bank filtration. On the other hand,some mobile and persistent organic micropollutants show a lower removal tendency.However, the compounds found in bank filtrate raw waters can in most cases be totallyremoved by subsequent treatment steps, like ozonation or GAC filtration.
0
10
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40
50
60
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Rem
oval
Effi
cien
cy in
%
Atr
azin
e
Sim
azin
e
Isop
rotu
ron
Diu
ron
Ter
buth
ylaz
ine
Chl
orto
luro
n
Dic
hlor
prop
Lind
ane
MC
PA
Met
habe
nzth
iazu
ron
source: Zullei-Seibert (1996)
DVGW-Water Technology Center (TZW), Germany Schmidt CK et al. (2003)
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Fig. 11. Removal efficiency of riverbank filtration for micropollutants at the lower Rhine.
Fig. 12. Behavior of sulfamethoxazol and amidotrizoic acid during aerobic and anaerobic bank filtration[OW: observation well; increasing numbers indicate higher flow path lengths]
18. Jun
20. Jun25. Jun
30. Jun9. Jul
14. Jul
18. Jul
23. Jul
28. Jul1. Aug
6. Aug
11. Aug
15. Aug
20. Aug
0
150
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450
600
c in
ng
/L
18. Jun
20. Jun25. Jun
30. Jun9. Jul
14. Jul
18. Jul
23. Jul
28. Jul1. Aug
6. Aug
11. Aug
15. Aug
20. Aug
0
10
20
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c in
ng
/L
RhineOW1
OW2OW3
RhineOW1
OW2OW3
River Bank filtrate
aerobic aquifer at the lower Rhine anaerobic aquifer at the Ruhr
Amidotrizoic acid
SulfamethoxazolSulfamethoxazol
Amidotrizoic acid
0
10
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50
60
70
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100
2-A
min
o-1,
5-na
phth
alen
edis
ulfo
nate
N-M
ethy
l-N-(
phen
ylsu
lfony
l)gly
cine
AO
X
AO
S
IOS
DO
C
SA
K
Car
bam
azep
ine
Am
idot
rizoi
c ac
id
Uro
trop
ine
Sul
fam
etho
xazo
l
1,5-
Nap
htha
lene
disu
lfona
te
Tris
(2-c
hlor
opro
pyl)p
hosp
hate
Tris
(2-c
hlor
oeth
yl)
phos
phat
e
MT
BE
Iopa
mid
ol
ED
TA
1,3,
6-N
apht
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netr
isul
fona
te
Trie
thyl
phos
phat
e
1-N
apht
hale
nesu
lfona
te
Tric
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oace
tic a
cid
Tris
(n-b
utyl
)pho
spha
te
Met
opro
lol
Dic
lofe
nac
Sot
alol
Clin
dam
ycin
e
Deh
ydra
to-E
ryth
rom
ycin
e A
Iopr
omid
NT
A
Iom
epro
l
Bis
phen
ol A
Non
ylph
enol
Rem
oval
Effi
cien
cy in
%
Bez
afib
rate
DVGW-Water Technology Center (TZW), Germany Schmidt CK et al. (2003)
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0.0
0.5
1.0
1.5
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
amm
onia
, man
gane
se [m
g/L]
0.0
1.0
2.0
3.0
4.0
oxyg
en [m
g/L]
ammonia
oxygen
manganesesource: ARW (1998)
Detailed studies demonstrated, that levels of many organic micropollutants present in Germanrivers can be reduced or even eliminated during both aerobic and anaerobic undergroundpassages. However, the elimination of others turned out to be clearly dependent on theunderlying redox processes in the groundwater. Figure 12 demonstrates, how sulfamethoxazol(an antibiotic) and amidotrizoic acid (a X-ray contrast agent) are well eliminated in ananaerobic aquifer, but are only slightly reduced during an aerobic underground passage.
Due to their physico-chemical properties lipophilic industrial chemicals and pesticides likeDDT or heptachlor are mostly sufficiently reduced by sorption processes at inorganic andorganic soil materials. Another point of recent concern are cyanobacteria and their toxins thatcan adversely affect water quality, especially in summer during algae bloom. However,artificial groundwater recharge and bank filtration result in an efficient removal ofcyanobacterial toxins and cells, except in very massive bloom situations.
Further Aspects
Treatment steps based on an underground passage can significantly lower the concentrationsof many surface-water pollutants; however, precise predicting and quantifying thosereductions is often difficult, since the efficiency of the underground passage depends onseveral factors. These include the river water quality, geological conditions, porosity of thesoil, residence time of the water in the soil, temperature, pH-conditions, and oxygenconcentration. Thus, the behavior of chemicals and microorganisms during infiltration andunderground passage of water depends on many different interacting factors. In general,however, the efficiency of underground passage is such that water quality is significantlyimproved.
Fig. 13. Development of bank filtrate quality at the Rhine River.
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Characteristics of the bank filtrate are affected by changes of the surface water quality that ischaracterized by the number of particles, concentration of dissolved organic matter fromnatural and artificial sources, oxygen, ammonia, nutrients, microorganisms, and otherpollutants. The Rhine River is an excellent example how changes in surface water qualityinfluence the characteristics of the corresponding bank filtrate. Figure 13 summarizes theconcentrations of ammonia, manganese, and oxygen in the bank filtrate of the Rhine Riverover a period of several years.
In the early 1970s, Rhine water was highly polluted. Ammonia was present and nearly nooxygen. Due to the reduction of biodegradable organic material during infiltration the littleoxygen present in the surface water and even nitrate were consumed, the aquifer wascharacterized by an anaerobic redox status, in which iron and manganese were reduced andreleased from the soil. In the mid 1980s, Rhine water quality improved because of bettermunicipal and industrial wastewater treatment and its oxygen concentrations increased. As aconsequence, conditions in the aquifer became aerobic, iron and manganese stayed in theinsoluble oxidized form (Fe3+, Mn4+) and finally disappeared in the bank filtrate.
Another example demonstrating the direct dependency of the bank filtrate condition on thequality of the surface water is the long-lasting development of AOX concentrations (figure14). The term AOX (adsorbable organic halogens) means the amount of organic halogenspresent in water. In the 1970s and early 1980s, high AOX concentrations were found in theRhine water. Over the years, paper mills replaced chlorine bleaching by oxygen treatment dueto the pressure of drinking water suppliers and, as a consequence, AOX levels in the Rhinedropped. It is obvious how the efforts to reduce the AOX levels in the Rhine during the lastyears resulted also in a higher bank filtrate quality.
Fig. 14. Development of AOX concentrations in river water and bank filtrate.
0
20
40
60
80
1975 1980 1985 1990 1995 2000
AO
X [µ
g/L]
river Rhine
bank filtrate
source: ARW (1998)
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Benefits
Based upon the above discussions, it is clear that river bank filtration and artificial infiltrationcan help utilities in various ways. Both techniques provide several advantages, such as theprotection against shock loads, temperature equalization, and the removal of particles,biological contaminants and biodegradable compounds. Underground passage is a verynatural step and is able to replace and specially to support other treatment steps. Theunderground passage improves the drinking water quality and makes drinking water safer andmore acceptable for the consumer. Thus, one of the more unrecognized values of undergroundpassage are avoided medical cost and longer life span.
When looking at the various types of purification processes intentionally allowed to occur inthe aquifer during bank filtration and artificial infiltration, the question inevitably ariseswhether these processes will be exhausted or whether the underground passage can be usedcontinuously. In Germany, bank filtration and artificial groundwater recharge have been usedfor the drinking water supply at several sites making use of different subsoil characteristicsfor decades and no loss of purification capacity could be noticed. This was also confirmed byseveral intensive investigations concerning this aspect. Despite considerable qualityfluctuations of surface waters, natural processes did always (even following temporarilymassive interruptions) turn back to normal. Furthermore, no alarming accumulation ofpersistent pollutants could be ascertained by investigation of various subsoils used forunderground passages.
The costs for establishing riverbank filtration or artificial groundwater recharge systemsdepend on many factors, including aquifer characteristics, type of well-screen installation,facility design, and distance to the population served. However, costs can be classified asmoderate.
Conclusions
Riverbank filtration and artificial groundwater recharge are well established techniques inGermany and are most often used as an important component of the established multiplebarrier system. Passage of water underground provides several benefits for drinking watertreatment. Experience demonstrates that during infiltration and underground transport,processes such as filtration, sorption, and biodegradation produce significant improvements inraw water quality. Underground passage as water treatment procedure combines particleremoval, pathogen removal, organic and inorganic chemical removal, peak smoothing inspills, temperature equalization, reduction in DBP formation, and production of a morebiologically stable water. However, polar, persistent organic substances are often notcompletely removed during underground passage. Elimination rates of these substances varywith residence time and length of the subsoil passage and sometimes depend on the redox
DVGW-Water Technology Center (TZW), Germany Schmidt CK et al. (2003)
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status. This deficiency is well known; therefore, many water-treatment systems rely onadditional treatment barriers such as oxidation and adsorption.
However, on the basis of a comprehensive evaluation of the available data material, it isobvious that the water quality is improved and subsequent treatment steps may be supportedand simplified leading to decreased water treatment costs. Due to the significant reduction ofDOC concentrations the run-time of activated carbon filters can be extended. Since bankfiltration or infiltration remove biodegradable substances naturally, residuals requiring watertreatment are lowered and less chemicals are necessary in subsequent flocculation andoxidation steps. Furthermore, the removal of particles and microorganisms during bankfiltration supports other treatment steps such as filtration, membrane technologies, ordisinfection.
References
Arbeitsgemeinschaft Rhein-Wasserwerke e.V. (ARW), 55. Jahresbericht 1998, Köln, ARW (1999)
BMI - Bundesministerium des Inneren (Ed.), Uferfiltration – Bericht des BMI-Fachausschusses“Wasserversorgung und Uferfiltrat”, Bonn (1975)
BMI - Bundesministerium des Inneren (Ed.), Künstliche Grundwasseranreicherung – Stand der Technik und desWissens in der Bunderepublik Deutschland, Berlin, Erich Schmidt Verlag (1985)
Brauch H-J, Sacher F, Denecke E, Tacke T, Wirksamkeit der Uferfiltration für die Entfernung von polarenorganischen Spurenstoffen, gwf Wasser Abwasser (2000) 141(4), 226-234
Brauch H-J, Müller U, Kühn W, Experiences with riverbank filtration in Germany, Proceedings, International
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